High-intensity focused ultrasonic energy (i.e., having a frequency greater than about 20 kilohertz) may be used therapeutically to treat internal tissue regions within a patient. For example, ultrasonic waves may be used to induce coagulation and/or necrosis in a target tissue region, such as a tumor. In this process, the ultrasonic energy is absorbed by the tissue, causing the generation of heat. The absorbed energy heats the tissue cells in the target region to temperatures that exceed protein denaturation thresholds, usually above 60° C., resulting in coagulation, necrosis, and/or ablation of the tissue in the target region. Ultrasound can also be used for a variety of other treatment modalities, including, e.g., cavitation induced by ultrasound, neuromodulation, or controlled hyperthermia.
Focused-ultrasound methods may utilize, for example, a piezo-ceramic transducer that is placed externally to the patient, but in close proximity to the target. The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves (a process hereinafter referred to as “sonication”). The transducer may be shaped so that the waves converge in a focal zone. Further, the transducer is typically defined by a plurality of individually driven transducer elements whose phases and amplitudes can each be controlled independently from one another and, thus, can be set so as to result in constructive interference of the individual acoustic waves in the focal zone. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases between the transducer elements.
Focused-ultrasound treatment procedures typically require moving the focus across the target to cover the treatment area or volume, which is generally larger than the focal zone, in a series of sonications. In doing so, it is important to apply a sufficient amount of energy across the target to achieve the desired therapeutic effect while limiting accumulated energy levels in surrounding non-target tissues to avoid damage thereto. This requires detailed knowledge of the patient's anatomy in the region surrounding the target, as may be acquired, e.g., by magnetic resonance imaging (MRI). Further, it requires computational facilities to determine accurate and precise phase and amplitude settings for the individual transducer elements to focus the beam at the desired places. In many treatment scenarios, anatomical barriers, such as tissues that do not transmit or are highly sensitive to ultrasound, are located between the transducer array and the target, requiring the transducer to be either moved or driven with fewer than all elements, which further complicates the procedure. As a result, therapeutic procedures often reach a level of complexity that requires detailed advance planning to determine, based on the anticipated relative arrangement between the patient and the transducer, the phase and amplitude settings of all transducer elements as a function of time.
Conventional treatment-planning methods typically utilize a limited number of predefined sonication protocols corresponding to differently shaped focal zones, and involve “tiling” the treatment region with these shapes. Such methods, however, usually fail to account for the accumulated effect of overlapping “tiles,” ultrasound absorption outside the focal zone, as well as heat transfer from the focal zone into surrounding areas. For example, to plan treatment of a three-dimensional target volume, a conventional treatment planner might slice the volume into a series of adjacent two-dimensional slices, cover each slice by a number of predefined sonications, and compute the energy for each sonication based on the desired dose in the respective slice. During treatment, however, each slice absorbs energy not only from sonications focused at the slice, but also from sonications directed at neighboring slices. Consequently, the total amount of energy deposited in the slice may exceed that previously calculated. While such excessive heating may not have a detrimental effect for treatment of the target itself (e.g., because the desired treatment effect is target ablation), it may cause surrounding non-target tissues to be exposed to an unnecessarily high amount of ultrasound energy. Accordingly, it is desirable to add up all contributions to the heating of any region in order to set the energy delivered per sonication to a level no larger than necessary to effect treatment.
To some extent, a previously planned treatment procedure can be corrected during treatment based on real-time information about the treatment effect. MRI, for example, is not only useful to visualize the focus and/or target in order to guide the ultrasound beam, but can also be employed in various thermometry techniques to monitor the temperature distribution in a region including the target to ensure that it remains at a desired level or within a desired range (e.g., above an efficacy threshold in the target region and below a safety threshold in non-target tissues). If the temperature in the target is too low, additional sonications may be applied to reach the desired efficacy threshold. Conversely, if the temperature in a non-target region, or a target region to be treated nondestructively (e.g., for palliative purposes, or for controlled hyperthermia), is too high, a waiting period may be introduced to allow the tissue to cool off. In some circumstances, however, irreversible damage may already have been done following absorption of too much energy by a non-target tissue. Further, any adjustments to the sonication procedure during treatment based on a measured effect result in prolongation of the overall treatment time, which may not only strain the patient's patience, but also introduce errors, e.g., due to inadvertent but inevitable patient movements.
Accordingly, there is a need for systems and methods that facilitate more accurate planning of focused-ultrasound procedures for complex treatment scenarios including multiple clinical and anatomical constraints, taking into account ultrasound absorption outside the desired target region and heat transfer across the tissue.